1. Field of the Invention
The invention relates to semiconductor lasers and in particular to mode locked semiconductor lasers having an external resonant cavity.
2. Description of the Prior Art
Ultrashort optical pulses have found broad applications in electrooptic sampling, broad-band submillimeter-wave generation, optical computing, and other areas of optoeletronics. Mode locked semiconductor lasers in particular are compact sources of ultrashort pulses. Passive and hybrid mode locking has been employed to generate sub-picosecond pulses in semiconductor lasers. The saturable absorber used in a passive or hybrid mode locked semiconductor laser needs to satisfy the following requirements: (1) the absorber should saturate faster than the gain media; and (2) the recovery time of the saturable absorber should be faster than that of the gain media.
Two main kinds of semiconductor saturable absorbers have been investigated for passive mode locking: (1) proton-bombarded semiconductors; and (2) semiconductor quantum wells. The quantum-well absorbers, whose absorption saturation is due to the screening of excitons by free carriers, are very attractive for passive mode locking because they are inexpensive, compact, cover a wide wavelength range, and have fast response time. See, L. R. Brovelli, I. D. Jung, D. Kopf, M. Kamp, M. Moser, F. X. Kartner, and U. Keller, xe2x80x9cSelfstarting Soliton Mode locked Ti-Sapphire Laser Using A Thin Semiconductor Saturable Absorberxe2x80x9d, Electron. Lett. Vol. 31, pp.287, 1995. Recently, a low-loss, epitaxially grown semiconductor saturable Bragg reflector (SBR) was demonstrated to be a powerful saturable absorber for passive mode locking. See, L. R. Brovelli, supra; S. Tusda, W. H. Knox, S. T. Cundiff, W. Y. Jan, and J. E. Cunningham, xe2x80x9cMode-Locking Ultrafast Solid-State Lasers With Saturable Bragg Reflectorsxe2x80x9d IEEE J. of Selected Topics in Quantum Electron. Vol.2, pp. 454, 1996; S. Tusda, W. H. Knox, E. A. de Souza, W. Y. Jan, and J. E. Cunningham, xe2x80x9cLow-Loss Intracavity Alas/Algaas Saturable Bragg Reflector For Femtosecond Mode Locking In Solidstate Lasersxe2x80x9d, Opt. Lett., Vol.20, pp. 1406, 1996; B. C. Collings, J. B. Stark, S. Tsuda, W. H. Knox, J. E. Cunningham, W. Y. Jan, and R. Pathak, xe2x80x9cSaturable Bragg Reflector Self-Starting Passive Mode Locking Of A Cr4+:Yag Laser Pumped With A Diode-Pumped Nd:YVO4 Laserxe2x80x9d Opt. Lett., Vol.21, pp. 171, 1996; S. Tusda, W. H. Knox, and S. T. Cundiff, xe2x80x9cHigh Efficiency Diode Pumping Of A Saturable Bragg Reflector-Mode-Locked Cr:Lisaf Femtosecond Laserxe2x80x9d Appl. Phys. Lett. Vol.69, pp. 1538, 1996; and W. H. Loh, D. Atkinson, P. R. Morkel, M. Hopkinson, A. Rivers, A. J. Seeds, and D. N. Payne, xe2x80x9cPassive Mode-Locked Er3+ Fiber Laser Using A Semiconductor Nonlinear Mirrorxe2x80x9d, IEEE Photon. Technol. Lett., Vol. 5, 35, 1993. The saturable Bragg reflector consists of semiconductor quantum wells embedded in a Bragg reflector and functions as a nonlinear mirror for saturable absorption. This design reduces the losses introduced in the cavity and increases the saturation intensity and the damaging threshold. Femtosecond pulses have been generated using saturable Bragg reflector in passive solid-state lasers and fiber lasers as described in certain ones of the above references.
Although there has been progressed by using the solid state lasers with saturable Bragg reflector, semiconductor optical sources have some special advantages. For example, the semiconductor lasers provide the advantages for high quantum efficiency, electrically pumped and high repetition frequency in the optical communication system. To date, the semiconductor lasers combined with the saturable Bragg reflector have, however, not been studied or understood.
Monolithic mode-locked semiconductor lasers generating 100 fs optical pulses with high pulse energy play an important role for high-bit rate time-division multiplexed (TDM) systems. Though such short optical pulses have been demonstrated in semiconductor gain medium using external cavities and additional pulse compression, those lasers are very bulky and not suitable for practical applications. Monolithic mode-locked semiconductor lasers are compact, light weight, energy efficient, and do not require optical alignment. Though very impressive performance (600 fs pulse width, 350 GHz repetition frequency) has been demonstrated, the pulse energy of this type of multiple contact quantum well lasers is limited by the intra-cavity saturable absorber (xcx9c10 fJ). Such energy is not sufficient for most all-optical switching/demultiplexing systems.
What is needed is an apparatus and method for the generation of short optical pulses in external cavity mode-locked semiconductor lasers with saturable Bragg reflector.
What is further needed is an apparatus and method for effective in broadening the mode-locked spectrum and reducing the pulses width.
The invention is a laser capable of generating short pulses of less than 1000 femtoseconds comprising a semiconductor laser. And a resonant optical cavity having a reflecting mirror in which the mirror comprises a saturable Bragg reflector.
In the illustrated embodiment the semiconductor laser comprises a InGaAs/InGaAsP/InP buried heterostructure multiple quantum well laser fabricated using organometallic vapor phase epitaxy.
In one embodiment the resonant optical cavity is an external cavity, while in a second embodiment it is an internal cavity. The internal cavity includes an antiresonant Fabry-Perot saturable absorber disposed between a pair of diffraction Bragg reflectors within the internal cavity. The laser further comprises an inclined monolithic mirror to direct light to the antiresonant Fabry-Perot saturable absorber within the internal cavity.
In the illustrated embodiment the laser includes a basal substrate. The antiresonant Fabry-Perot saturable absorber is disposed on the substrate and the semiconductor laser is disposed on the antiresonant Fabry-Perot saturable absorber. Alternatively, the semiconductor laser may be disposed on the substrate and the antiresonant Fabry-Perot saturable absorber disposed on the semiconductor laser.
The saturable Bragg reflector is comprised of substrate, a Bragg stack disposed on the substrate and a multiple quantum well disposed on the Bragg stack. The substrate is composed of GaAs. The Bragg stack is comprised of multiple layers of GaAs/AlAs. The multiple quantum well is comprised of multiple layers of InGaAs/InGaAsP.
The laser further comprises a dispersive optic fiber optically coupled to the semiconductor laser for receiving and transmitting output therefrom to reduce frequency chirp. The optic fiber has a length which has been selected to minimize pulse width of the output from the semiconductor laser.
The invention is also a method of generating short laser pulses of less than 1000 femtoseconds in a mode locked laser comprising the steps of providing a semiconductor laser; providing a resonant external optical cavity having a reflecting mirror the mirror comprising a saturable Bragg reflector; coupling the resonant optical cavity in alignment with the semiconductor laser; adjusting external cavity alignment reverse-bias voltage of an on-chip saturable absorber in the semiconductor laser and forward gain currents of a gain section in the semiconductor laser to obtain a stable optical pulse output. The forward gain currents and reverse-bias voltage are biased to minimize pulse width of the stable optical pulse output.
The invention now having been briefly summarized, turn the following drawings, wherein like numerals reference like elements, and where the summarized invention is illustrated.